OK, I've cleaned up your drawing some, and made it a little more compact and easier to brace. I take it that there's no differential involved... 

Blaze2's design: 


the green axle is powered and spins the green 24t gear 
depending on the direction of rotation of the green 24t, the clear 24t gear is forced up or down taking the lever of the pneumatic switch with it 
the pump is attached to the other end of the green axle. 
This design works fine so long as the clear 24t doesn't get pushed too far up or down -- occasionally this happens and you have to reset the switch manually (so don't bury it inside the robot). 

Cheers 

JP